1. Field of the Invention
The present invention relates to a high pressure discharge lamp such as a sodium-vapor lamp, metal halide lamp or the like.
2. Background Art
A conventional high pressure discharge lamp is shown in FIG. 1, and includes a vessel 1 made of a non-conductive material (e.g. alumina) which forms an inner space filled with an ionizable light-emitting material and a starting gas. A tubular capillary member 2 is arranged at one opening portion of the vessel, and has an outer diameter which is substantially the same as an inner diameter of the first opening portion. An electrode unit 3 is inserted into the capillary member 2 and has an outer diameter which is smaller than an inner diameter of the capillary member 2.
In such a discharge lamp, a gap formed between the inner surface of the capillary member 2 and the outer surface of the electrode unit 3 is filled with a frit seal 4. However, as shown in FIG. 1, there may occur fluctuation of the axial position of the frit seal 4 in the capillary member 2, as represented by xe2x80x9cdxe2x80x9d in FIG. 1, so that the axially inner end of the frit seal is not uniformly positioned around the electrode unit. Such fluctuation makes it difficult to maintain a substantially constant volume of the ionizable light-emitting material and the starting gas flowing into the capillary 2, and to realize a uniform property of the discharge lamp. Moreover, when a corrosive material is used as the ionizable light-emitting material, the tendency of the corrosiveness of the electrode unit 3 is notable if the electrode unit 3 is excessively exposed to the discharge space of the vessel 1.
To avoid the above-mentioned fluctuation relating to the non-uniform positioning of the frit seal within the capillary member, it would be necessary to control the volume and the viscosity (i.e. temperature) of the frit seal, though such control is often difficult to perform in a practical manner.
It is an object of the present invention to provide a high pressure discharge lamp in which the axially inner end of the frit seal is uniformly positioned around the electrode unit without the need to control the volume and the viscosity of the frit seal.
According to the present invention, there is provided a high pressure discharge lamp comprising: a vessel made of a non-conductive material which forms an inner space filled with an ionizable light-emitting material and a starting gas, the vessel having an opening portion at one end thereof; a tubular member arranged at the opening portion of the vessel and having an outer diameter which is substantially the same as an inner diameter of the first opening portion; an electrode unit inserted into the tubular member and having an outer diameter which is smaller than an inner diameter of the tubular member; a stopper arranged between the tubular member and the electrode unit and having an outer diameter which is substantially the same as the inner diameter of the tubular member, the stopper having a hole which is inserted by the electrode unit; and a frit seal filled in a gap which is formed by the tubular member, the stopper and the electrode unit, with the stopper defining an inner end position of the frit seal in the tubular member.
According to the invention, a frit seal is filled in a gap which is formed by the tubular member, the stopper and the electrode unit, with the stopper defining an inner end position of the frit seal in the tubular member. The axially inner end of the frit seal can be uniformly positioned around the electrode unit by the stopper without the control of the temperature of the frit seal.
Preferably, the stopper comprises a porous non-conductive member.
Composing the stopper of a porous non-conductive member eliminates the need to control the volume of the frit seal. The axially inner end of the frit seal can be uniformly positioned around the electrode unit because excess frit seal is absorbed by the porous non-conductive member. In this connection, the porous material which can be suitable used in the present invention has a number of pores with an average pore diameter of approximately 1 to 10 xcexcm and a porosity of not less than approximately 30%.